<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>  </title>
    <style>
        .box{
            width: 300px;
            height: 300px;
            background-color: red;
            margin: 50px auto;
            /* border-radius: 300px 50px; */
            border-bottom-right-radius: 300px; 
        }
        .box2{
            height: 200px;
            width: 200px;
            background-color:black;
            margin:40px auto;
            border:100px solid white;
            box-sizing: border-box;
            /* border-color: red blue yellow green; */
            
        }
    </style>
</head>
<body>
    <div class="box"></div>
    <div class="box2"></div>
</body>
</html>